All-wave loop receiver



y 1941- D. GRIMES 2,250,370

' ALL-WAVE LOOP RECEIVER Filed June 3, 1939 2 Sheets-Sheet l i: 1 I L DM 69m;

July 22, 1941. .D. GRIMES ALL-WAVE LOOP RECEIVER Filed June 3, 1 939 2 Sheets-Sheet 2 Patented July 22, 1941 grasp r or ies ALL-WAVE LOOP nnoinvnn David Grimes, Beverly, N. 3., assignor, by mcsne assignments, to Philco Radio and Television Corporation, Philadelphia, Pa, a corporation of Delaware Application .lune 3, 1939, Serial No. 277,276

12 Claims. (Cl. 256-20) This invention relates to loop antennas or aerials, and more particularly to a combination of loop antennas capable of adapting a radio receiver to loop reception over a plurality of wavebands such as the broadcast band and one or more short-wave bands. The invention also contemplates the provision of certain circuits adapted to be associated with the loop antennas and/or the radio receiver, and intended to improve the efficiency and operation of the receiver.

In order to improve the portability of radio receivers and to simplify their installation, it is desirable to employ a tuned loop antenna as a means for abstracting energy from passing carrier wave signals, in place of the usual outdoor antenna structure. Such a loop antenna for use, for example, in the broadcast band is fully de scribed in the copending application of Wm. H. Grimditch Serial No. 277,550, filed June 5, 1939. In a modern radio receiver it is desirable, however, to provide means for receiving broadcasts in a plurality of wave bands, such as the long wave, broadcast, police, and short-wave bands. This necessitates the utilization either of a plurality of loops or of a tapped loop. Where such a plurality of loops are placed Within the confines of the usual radio receiver cabinet or console, the physical separation between the loops is necessarily small and hence the danger of undesired coupling between the several loops is great. Such coupling is especially to be avoided in the case of short wave reception. Suppose that during such reception the longer wave loop or loops are removed from the receiver input circuits and either short-circuited or open-circuited. In either case, the distributed capacities associated therewith will normally tend, in combination with the loop inductances, to resonate at some frequency or frequencies in the short-wave band. At these frequencies the resonant openor short-circuited unused loops will act as a substantial load on the short-Wave loop being utilized, thereby efiectively reducing the Q of the short wave loop, and hence reducing the voltage which said loop can supply to the receiver input.

According to the present invention, the above difficulties are surmounted and additional advantages are obtained by a novel arrangement of the loops. It has been found that although it is normally essential that the broadcast or long- Wave loops be mounted in vertical planes, it is possible to secure equal 01' even better short-wave reception by placing the short-wave loop in a horizontal plane. Two desirable ends are thereby accomplished; firstly, the coupling between the wave stations loops may be reduced to a minimum, and secondly, the short-wave loop becomes non-directional, making it unnecessary to rotate the short-wave loop (or the receiver cabinet) as different shortare tuned in. The directional properties of the broadcast loop are retained however, and this is in general desirable in view of the greater crowding of stations and of the greater local interference encountered in the broadcast band.

One object of this invention, therefore, is to provide, in combination with a radio receiver, a plurality of loop antennas capable of adapting the said receiver to emcient operation in a plurality of wave bands.

Another object of the invention is to provide,-

within a radio receiver cabinet or console, a combination of a long-Wave loop and a short-wave loop, so positioned that the mutual inductance therebetween shall be a minimum.

Still another object of the invention is the provision of a directional broadcast loop in combination with a non-directional short-wave loop, so oriented that the undesired coupling therebetween is reduced to a negligible factor.

A further object of the invention is to provide a simple and economical construction for a short wave loop adapted for association with a broadcast or long-wave loop as contemplated by the invention.

A still further object of the invention is to provide certain receiver circuits for improving the operation of receivers employing loop antennas.

In the drawings:

Fig. 1 is a diagrammatic perspective illustration of the relative disposition of the long-wave and short-wave loop antennas according to the invention;

Fig. 2 is a face view of a short wave loop constructed in accordance with the invention; and

Fig. 3 is a schematic representation of a portion of an all-Wave superheterodyne radio receiver employing the loop antennas of the invention.

Referring now to Fig. 1, there is shown associated with a radio receiver represented generally by its chassis I, a broadcast-band loop antenna 2 disposed in a vertical plane, and a short-Wave loop antenna 3 disposed in a horizontal plane. The planes of the loops are therefore mutually perpendicular, and it will be understood that as a result, the inductive coupling therebetween will be considerably less than if the loops were disposed in parallel planes. The coupling may be reduced to substantially zero by arranging the loops so that the plane of the broadcast loop 2 exactly bisects the short wave loop 3, as shown by the dotted lines, the line of intersection being designated .r--ac. Alternatively, the plane of the loop 3 might be arranged to bisect the plane of the loop 2. These optimum relations of the two loops cannot always be achieved precisely in small cabinet type receivers, but preferably the perpendicular relation between the planes of the loops should be maintained, thus giving less coupling between the loops than if the loops were disposed in closely adjacent parallel planes, and also providing optimum conditions for reception in the several wave bands, as hereinbefore mentioned.

The reduction of coupling between the loops is particularly important during short-wave reception. Suppose that the short-wave loop 3 is tuned to resonance with the desired transmission by means of a tuning condenser connected across its terminals 4, and that the broadcast loop 2 is disconnected from the said tuning condenser with its terminals 5 either openor short-circuited. Because of the relatively large number of turns which comprise the loop 2, and because of the relatively large distributed capacity of the loop 2 which exists between the turns of the loop and from the loop to ground (e. g. the chassis I), it will be obvious that the inductance of the loop may resonate with the various distributed capacities at one or more frequencies Within a higher frequency band, e. g. the band over which the short-wave loop 3 is tunable. Such a condition will result in a marked reduction of apparent receiver sensitivity at the frequencies over which the unused loop is resonant. By reducing the coupling between the loops, as hereinbefore described, this undesirable effect can be eliminated entirely or at least reduced to a point where it is of no practical importance.

It has been stated that the broadcast loop 2 should be disposed preferably in a vertical plane. This follows from the well known fact that wave signals of broadcast frequencies are vertically polarized, that is the vector representing the magnetic component of the wave signal lies in a substantially horizontal plane. It is, of course, assumed that the distance between the receiving loop and the transmitting antenna is substantially greater than one wave length. Since a loop antenna is primarily responsive only to the magnetic component of the desired electromagnetic wave signal, it is obvious that for maximum signal pick-up in the broadcast band the plane of the loop must be vertical.

It has been found, however, that in the reception of short Waves it is possible to obtain equal and even better reception by disposing the short Wave loop in a horizontal plane. This is evidently due to the fact that at considerable distances from the transmitter, short wave signals may possess both horizontally and vertically polarized components. In fact, in some instances the vertically polarized components may be almost absent. Of further importance is the fact that the angle of arrival of these short waves is generally less than 45 degrees above the horizon. It is to be noted that these relations hold whether the transmitting antenna originally radiated a horizontally or a vertically polarized wave. The effect appears to be due to certain phenomena in volving the Kennelly-Heaviside layer and the earths magnetic field.

In accordance with the present invention, advantage is taken of this difference in the plane of polarization of passing long-waves and shortwaves, and this difference is utilized and turned to advantage by providing longand short-wave loops in vertical and horizontal planes, respectively, thus not only improving reception in each of the respective bands, but also reducing the degree of undesired inductive coupling between the loops, hereinbefore explained.

In addition to the above advantages residing in the combination of a horizontal short-Wave loop and a vertical long-Wave loop, it has been observed that the noise pick-up of a horizontal short-wave loop is considerably less than that of a similarly designed vertical loop. This difference is due to the fact that the more severe short-Wave noise signals are generated locally and are vertically polarized, and are thus greatly discriminated against by the horizontal loop described herein.

In the application of the invention to table model radio receivers and the like, the broadcast loop 2 may be positioned near the rear edge of the chassis I, while the short-wave loop 3 may be positioned above the chassis, for example by fastening it to the under surface of the top of the cabinet or casing which houses the receiver.

A preferred form of short wave loop construction is illustrated in Fig.2. When employed with a standard type of tuning condenser to tune over the short Wave band extending approximately from 6 to 18 megacycles, the loop may comprise roughly a single turn of wire 8 wound on a relatively stiff sheet of insulating material 1. The sheet l may be shaped as illustrated, having end portions 8 and notches 9, and the wire 6 may pass from one face or side of the sheet to the other, about the ends and through the notches. A pair of terminals l0 and II may be riveted or otherwise fastened to the insulating sheet, and the leads I2 and i3 soldered thereto for making such connections to ground or to switches or tuning condensers as may be desired. The wire 6 is preferably of a kind having a low radio frequency resistance, so that the Q of the coil may be as high as possible. Enameled copper wire, No. 20 or larger, has been used with satisfactory results.

Attention is now directed to Fig. 3, which illustrates an application of the loop antennas of the present invention to a portion of a radio receiver. In the particular illustration, a group of switches S1, S2, and S3 have been ganged to provide rcception over the broadcast band (position A) extending from about 550 to 1550 kc., the police band (position B) extending from about 1600 to 3500 kc, and the short-wave band (position C) extending from about 6 to 18 megacycles. In the broadcast position A, the short-wave loop 3 is preferably open circuited, while the ends of the broadcast loop 2 are connected across the tuning condenser i l by switches S1 and S3. In the police band position B, the portion of the broadcast loop between leads l6 and !5 may be short circuited and grounded by means of the switches S2 and S3, While the remaining portion of the loop between leads I5 and I! is connected across the tuning condenser I4 by means of the switches S1 and S2. In the short wave position C, the broadcast loop 2 is left open-circuited and ungrounded, while the ungrounded side of the short wave loop 3 is connected to the high potential side of the tuning condenser I4 by means of the switch S1. The broadcast loop 2 and the shortwave loop 3 are preferably in vertical and horizontal planes, respectively, as hereinbefore explained.

If it is desired to employ an outside antenna, the antenna may be coupled into the resonant loop circuit, in the police and broadcast bands, by connecting it to the terminal designated ANT which is coupled to one of the intermediate turns of the loop 2 by way of the shunt resistor-capacitor circuit I8 and the lead iii. In the short-wave band, coupling between the terminal ANT and the loop 3 may be by way of the capacitor 2! which may have a low capacity of perhaps micro-microfarads. I

Referring briefly to the remainder of the figure, there is shown a radio frequency amplifier tube V1 which is resistance coupled to a frequency converter or detector-oscillator tube V2. In order to improve, the transfer of energy at high frequencies across this untuned resistance coupling network, which comprises the resistors 2| and 22, and the capacitor 23, a resistancedamped peaking coil 24 is provided in the plate lead of the tube V1, and is adjusted to resonate with the input capacity of the tube V2 at some frequency in the short-wave band, say 9.5 megacycles. The use of an untuned radio frequency stage is desirable in that it eliminates the necessity of a band switching arrangement in that stage. An intermediate frequency amplifier V3 may be coupled to the output of the frequency converter V2 by means of a tuned transformer 25, while the second detector and AVC tube V4 may be coupled to the output of the amplifier V3 by means of a second tuned transformer 26.

Since the signal supplied to the input of a radio receiver by a loop antenna is substantially smaller than that provided by a well designed outdoor antenna of large dimensions, it has been found desirable to maintain a high gain ahead of the frequency converter tube V2 for all but the more powerful signals, for the reason that the converter tube has a relatively high inherent noise level. By supplying the converter tube with as large a signal as possible, the ratio of the desired signal voltage to inherent converter tube noise voltage (in the converter tube output) may be made high, thereby reducing the effect of the said noise. Consequently, it has been found advantageous to use a high mutual conductance radio frequency amplifier tube V1 ahead of the converter. The tube V1 is preferably one having a sharp cut-on characteristic, since such tubes have a lower inherent noise level (due to thermal effects, shot effects and the like) than do tubes of the remote cut-off or variable mu types. This is an important consideration where the signals involved are of low level such as those derived from loops. The tube V1 may then be supplied with a fraction of the total available AVC (gain control) voltage, for example by employing a voltage divider 21 connected across the second detector load resistance 28. Consequently, for very weak and average signals, the gain of the tube V1 will remain relatively high, whereas only for strong signals will the AVG voltage be sufiicient to greatly reduce the gain of the amplifier V1. On the other hand, the tube V3 employed in the intermediate frequency stage may be of the remote cut-off, or variable mu type, and it may be supplied with the maximum available AVC voltage obtained, for example, directly from the anode of the AVG tube V4 by way of the filter which comprises the resistor 2Q and the capacitor 30. Thus the gain of the intermediate frequency amplifier will vary widely over the range of signal amplitudes. If desired AVC voltage may also be applied to the gain controlling element of the tube V2.

Another method of maintaining a high gain in the radio frequency amplifier stage during weak signal reception is to provide separate AVC sources for the radio frequency and intermediate frequency amplifiers. The AVC source which supplies the gain controlling voltage to the radio frequency amplifier may be given an initial voltage delay of such a magnitude that no gain control voltage is generated until the input signals exceed a certain predetermined amplitude level. The AVC source which supplies the intermediate frequency amplifier may be given the usual small delay voltage, or none at all, as is well understood in the art.

It has been found that radio receivers employing a loop antenna, in which the loop is physically rather closely associated with various receiver components, exhibit a tendency toward undesired oscillations due to coupling between the intermediate frequency transformers and the loop. It has been found moreover that the usual shielding provided around the intermediate frequency transformers, while quite adequate with receivers equipped with the usual antenna and antenna transformer, is entirely insufficient to prevent this regenerative effect in receivers employing loop antennas. This effect may be avoided however by the use of double and spaced shields around the offending transformers. Such a double shield is indicated schematically at 3! and 32 about transformer 28. The use of a double shield is considerably more effective than a single shield, even though the single shield be of twice the usual thickness. The material of the shields may be of low permeability tinned steel, or of non-magnetic metals, such as copper or aluminum. If necessary both transformers 25 and 25 may be provided with double shields, and in severe instances the last intermediate transformer, which is normally the worst ofiender, may be so oriented as to produce a degenerative effect rather than a regenerative effect.

Although the invention has been described with particular reference to the embodiments of the drawings, it will be understood that the invention is capable of various forms of physical expression, and is not to be limited to the specific disclosure, but only by the scope of the appended claims.

I claim:

1. In a multi-band radio receiver, a receiver chassis, a long wave loop antenna disposed in a vertical plane near the rear of said chassis, a short Wave loop antenna disposed in a horizontal plane near the upper part of said long wave antenna and spaced from said chassis,

whereby the electrical influence of said chassis upon said loops is rendered substantially ineffective, and the loops are arranged for optimum reception in their respective frequency bands with minimum coupling between them, and means for selectively connecting said antennas to the receiver.

2. In a multi-band radio receiver, a plurality of loop antennas adapted respectively for the reception of signals in different frequency bands, said loop antennas being positioned within the mutual magnetic fields thereof, common tuning means for tuning said antennas to various signals within their respective frequency bands, and switching means for selectively connecting said antennas to said tuning means, the said loop antennas including a first loop adapted to receive signals in a high frequency band and disposed in a horizontal plane, and a second loop adapted to receive signals in a relatively low frequency band and disposed in a vertical plane.

3. In a multi-band radio receiver, a first loop antenna constructed and arranged for maximum response to vertically polarized low frequency wave signals, a second loop antenna constructed and arranged for maximum response to horizontally polarized high frequency wave signals, and tuning means for independently tuning said loop antennas over non-coextensive high and low frequency wave bands, respectively, each loop being positioned well within the local magnetic field set up by the other loop, but so oriented that the inductive coupling between the loops is negligible.

4. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a first loop antenna adapted to receive signals in a high frequency band and disposed in a horizontal plane within said cabinet, a second loop antenna adapted to receive signals in a relatively low frequency band and disposed in a vertical plane Within said cabinet, and means for selectively connecting said antennas to said receiver, the proximity of said loop antennas to one another due to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible.

5. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a first loop antenna adapted to receive signals in a high frequency band and disposed in a horizontal plane within said cabinet, a second loop antenna adapted to receive signals in a relatively low frequency band and disposed in a vertical plane within said cabinet, the said loop antennas being so arranged that the plane of one loop antenna bisects the other loop antenna, and means for selectively connecting said antennas to the'receiver, the proximity of said loop antennas to one another clue to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the cou pling between them is negligible.

6. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a directional long wave loop antenna within said cabinet, a non-directional short wave loop antenna within said cabinet and in proximity to said long wave antenna, and means for selectively connecting said antennas to said receiver, the proximity of said loop antennas to one another due to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible.

'7. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a directional long wave loop antenna comprising a plurality of turns and positioned within said cabinet, a non-directional short wave loop antenna comprising approximately a single turn and positioned within said cabinet, and switching means for selectively connecting said antennas to a single tuning means and to said receiver, the proximity of said loop antennas to one another due to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible.

8. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a single-turn loop antenna adapted to receive signals in a high frequency band and disposed in a substantially horizontal plane within said cabinet, and a multi-turn loop antenna adapted to receive signals in a relatively low frequency band and disposed in a substantially vertical plane within said cabinet, the proximity of said loop antennas to one another due to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible.

9. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a single-turn loop antenna adapted to receive signals in a high frequency band and disposed in a substantially horizontal plane within said cabinet, a multi-turn loop antenna adapted to receive signals in a relatively low frequency band and disposed in a substantially vertical plane within said cabinet, the proximity of said loop antennas to one another clue to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible, and switching means for selecting only a limited number of the turns of said multi-turn loop whereby the said p may be adapted to receive signals in a medium frequency band.

10. In a multi-band radio receiver including a cabinet or housing within which the receiver clements are disposed, a first loop antenna adapted to receive signals in a high frequency band and disposed in a horizontal plane within said cabinet, 2. second loop antenna adapted to receive signals in a relatively low frequency band and disposed in a vertical plane within said cabinet. said second loop having a central vertical axis, the proximity of said loops to one another clue to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible and substantially unaffected by the rotation of said second loop about said axis, and means for selectively con necting said antennas to said receiver.

11. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a first loop antenna adapted to receive signals in a high frequency band and disposed in a horizontal plane within said cabinet, a second loop antenna adapted to receive signals in a relatively low frequency band and disposed in a vertical plane within said cabinet, said second loop being substantially symmetrical about a vertical axis, said loop antennas being so arranged that one of said loops lies wholly outside the plane of the other of said loops, the proximity of said loop antennas to one another clue to their mutual disposition within said cabinet being a potential cause of undesired coupling between them, said loop antennas being so oriented that the coupling between them is negligible and substantially independent of the orientation of said second loop with respect to its vertical axis, and means for selectively connecting said antennas to said receiver.

12. In a multi-band radio receiver including a cabinet or housing within which the receiver elements are disposed, a first loop antenna Within said cabinet constructed and arranged for reception of signals in one frequency band, a second loop antenna Within said cabinet constructed and arranged for reception of signals in a difierent frequency band, said loops being disposed in nonparallel planes, one of said loop antennas comprising a stiff insulating sheet having cut-away 10 negligible.

portions and a single turn of wire extending on DAVID GRID/LES. 

